Engine developers and lubricant formulators are constantly improving the performance of internal combustions engines by reducing the power losses and emissions. The majority of the mechanical frictional losses generated in an engine can be attributed to the main tribological components of an engine, the valve train, piston assembly and engine bearings. However no single method has been developed to measure the friction loss contribution of each component simultaneously in a firing engine. Such results would be invaluable to the automotive/lubricant industries, research institutions and for validating predictive mathematical models for engine friction. The main focus of the research reported in this thesis was to validate an engine friction mathematical model called FLAME, developed in a separate study at Leeds. The validation was achieved by experimentally characterising the frictional losses generated from the major tribological components of a single cylinder gasoline engine. A novel experimental system was developed to evaluate experimentally, frictional losses in all the three main tribological components of an engine under fired conditions. A specially designed pulley torque transducer was used to measure valve train friction whereas improved IMEP method was adopted to measure piston assembly friction. For the very first time bearing friction was determined experimentally in a fired engine indirectly by measuring total engine friction. The FLAME engine friction model predicted valve train friction of the same order as the experimental data at engine speeds of 1500rpm and above. However, there was a much-reduced sensitivity to engine speed and temperature in the predictions. The piston assembly predicted results correlated very well with the measured data especially at lubricant inlet temperature of 80°C whereas for the bearing friction, the predicted results obtained with the short bearing approximation for the 1t film case were very close to the measured values. Overall the predicted total engine power loss results showed a good correlation with the experimental data especially at high lubricant inlet temperatures and engine speeds. It was concluded that the predicted results were in good agreement with the experimental results and the comparison validated the FLAME engine friction model.